It is a well known fact that a flexible membrane, e.g. a balloon, filled with pressurized air will tend, as nearly as possible given the structural constraints of the balloon, to assume a spherical shape. If a portion of the balloon's surface is forcibly displaced from the position of minimum energy, the pressure inside the balloon will increase. Under this principle, the pressure inside a person's eye may be controllably increased to a point where the diastolic pressure of the retinal artery is no longer great enough to sustain the artery's shape against the aqueous pressure in the eye. Under these circumstances, a pulsation occurs during the systolic pressure increase. If the pressure in the eye is further increased, the retinal artery collapses completely, even the systolic pressure in the artery being insufficient to counteract the pressure induced in the eye by the application of negative pressure to the sclera of the eye.
A device for diagnosing unilateral internal carotid occulsive disease includes a vacuum-generating system for applying negative pressure to the limbal area of a patient's eye via a suction eye cup. The pressure applied to the eye is read by means of a gauge. Thus, an examining physician must operate the vacuum generator and observe the patient's eye for the partial and complete collapse of the retinal artery. In the meantime he must monitor the gauge, or else have an assistant do it for him, and must note the pressure magnitudes at which the diastolic and systolic pressures are attained. Upon performing the test for both eyes, a series of tedious mathematical calculations must be carried out for determining diastolic and systolic negative-pressure differences between the left eye and the right eye. If the differences are of sufficient magnitude, i.e. greater than a predetermined magnitude, an occlusion is diagnosed in the internal carotid artery subsystem for which the negative pressures applied to the respective eye are greater than the pressures applied to the other eye. A more advanced form of this instrument is known in which the gauge is replaced by a digital display; the time required to collect data, however, is not substantially reduced and the calculations are not reduced.